Explicit Keywords on all music21 objects

[mscuthbert]

Fixes #839

Each push will include one or more modules.

Starting with Base.

[coveralls]

Coverage increased (+0.02%) to 93.038% when pulling 4cb3228 on explicit-keywords into 1a0ec50 on master.

[mscuthbert]

Misc fixes:

• Metadata date formats derive from DatePrimitive not DateSingle (attn: @gregchapman-dev)
• remove unused note.linkage
• kwargs -> keywords throughout
• fix metadata tests pinned to 8.0.0a9

Modules/Classes affected include:

• base
• Tuplet
• note
• Stream
• chordBase
• discrete analysis
• Variant (now takes only one arg)
• midi realtime, etc.
• Chorales, Date primitives, RepeatBracket
• layout